Page 474 - Environment: The Science Behind the Stories

areas in valleys ringed by mountains, from Tehran to Seoul to Together these three pairs of convective cells create wet
Río de Janeiro to São Paulo. climates near the equator, arid climates near 30° latitude,
moist regions near 60° latitude, and dry conditions near the
Large-scale circulation systems produce poles. These patterns, combined with temperature variation,
global climate patterns help explain why biomes tend to be arrayed in latitudinal
bands (see Figure 4.17, p. 111).
At large geographic scales, convective air currents contribute The Hadley, Ferrel, and polar cells interact with Earth’s
to broad climate patterns (Figure 17.9a). Near the equator, solar rotation to produce global wind patterns (Figure 17.9b). As
radiation sets in motion a pair of convective cells known as Earth rotates on its axis, locations on the equator spin faster
Hadley cells. Here, where sunlight is most intense, surface air than locations near the poles. This means that as air currents
warms, rises, and expands. As it does so, it releases moisture, of the convective cells flow north or south, some regions of the
producing the heavy rainfall that gives rise to tropical rainfor- planet’s surface move west to east beneath them more quickly
ests near the equator. After releasing much of its moisture, this than others. As a result, from the perspective of an Earth-
air diverges and moves in currents heading north and south. bound observer, these air currents appear to be deflected from
The air in these currents cools and descends at about 30 degrees a straight path. This deflection is called the Coriolis effect, and
latitude north and south. Because the descending air has low it results in the curving global wind patterns in Figure 17.9b.
relative humidity, the regions around 30 degrees latitude are Near the equator lies a region with few winds known as the dol-
quite arid, giving rise to deserts. Two further pairs of convec- drums. Between the equator and 30° latitude, trade winds blow
tive cells, Ferrel cells and polar cells, lift air and create precipi- from east to west. From 30° to 60° latitude, westerlies blow
tation around 60 degrees latitude north and south and cause air from west to east. People made use of these patterns for cen-
to descend at 30 degrees latitude and in the polar regions. turies to facilitate ocean travel by wind-powered sailing ships.

Ferrel cell Polar cell
60º N
Hadley cells

60º N (moist) Westerlies 30º N
30º N (arid)
NE trade winds

0º (Equator) (Doldrums) Equator
Ferrel cell (wet, tropical) SE trade winds
30º S (arid)
Polar cell 60º S (moist)
Westerlies 30º S

60º S CHAPTER 17 • AT m os PHER i C sC i E n CE , Ai R Qu A li T y, A nd Poll u T i on Con TR ol
Hadley Cell Hadley Cell

(b) Global wind patterns

Equator
30º S 30º N

Figure 17.9 Large-scale convective cells create global
patterns in moisture and wind. These cells (a) give rise to a wet
climate in tropical regions, arid climates around 30° latitude, moist
Arid Wet, tropical Arid climates around 60°, and dry climates near the poles. Surface air
climate climate climate
movement of these cells interacts with the Coriolis effect to create
(a) Convection currents (b) global wind currents. 473

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